Thermocouple-controlled catheter cooling system

ABSTRACT

The present invention provides a medical system, including a catheter defining an injection lumen, a chamber in fluid communication with the injection lumen, and an exhaust lumen in fluid communication with the chamber; a first temperature sensor positioned in the exhaust lumen proximal to the chamber; a second temperature sensor positioned in the chamber; and a console in electrical communication with the first and second temperature sensors, the controller modifying coolant flow through the medical device based at least in part upon a signal received from the first and second temperature sensor. The system may further include a thermally-conductive element circumscribing a substantial portion of the exhaust lumen proximal to the chamber, where the first temperature sensor is mounted to the thermally-conductive element, and the thermally-conductive element may include at least one of a braid, coil, and band.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of patent application Ser. No.12/122,436, filed May 16, 2008, entitled THERMOCOUPLE-CONTROLLEDCATHETER COOLING SYSTEM, the entirety of which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

n/a

TECHNICAL FIELD

The present invention relates to a method and system for controllingfluid flow in a medical device, and in particular, towards controllingthe flow of a coolant in an intravascular catheter via temperaturefeedback.

BACKGROUND

The use of fluids with low boiling temperatures, or cryogens, isbecoming increasingly explored and employed in the medical and surgicalfield. Of particular interest is the use of catheter based devicesemploying the flow of cryogenic fluids therein to selectively freeze orotherwise thermally affect targeted tissues within the body. Catheterbased devices are desirable for various medical and surgicalapplications in that they are relatively non-invasive and allow forprecise treatment of localized discrete tissues that are otherwiseinaccessible. Catheters may be easily inserted and navigated through theblood vessels and arteries, allowing non-invasive access to areas of thebody with relatively little trauma.

Catheter-based ablation systems are well known in the art. A cryogenicdevice uses the energy transfer derived from thermodynamic changesoccurring in the flow of a cryogen therethrough to create a net transferof heat flow from the target tissue to the device, typically achieved bycooling a portion of the device to very low temperature throughconductive and convective heat transfer between the cryogen and targettissue. The quality and magnitude of heat transfer is regulated by thedevice configuration and control of the cryogen flow regime within thedevice.

A cryogenic device uses the energy transfer derived from thermodynamicchanges occurring in the flow of a refrigerant through the device. Thisenergy transfer is then utilized to create a net transfer of heat flowfrom the target tissue to the device, typically achieved by cooling aportion of the device to very low temperature through conductive andconvective heat transfer between the refrigerant and target tissue.Structurally, cooling can be achieved through injection of high pressurerefrigerant through an orifice. Upon injection from the orifice, therefrigerant undergoes two primary thermodynamic changes: (i) adepressurization (adiabatic) and temperature drop through positiveJoule-Thomson throttling, and (ii) undergoing a phase change from liquidto vapor, as the fluid absorbs heat. The resultant flow of lowtemperature refrigerant through the device acts to absorb heat from thetarget tissue and thereby cool the tissue to the desired temperature.

Presently available cooling systems typically operate based upon a setcoolant flow rate or coolant pressure in the catheter or medical devicethat is used to reach a desired temperature for treatment. A measurementof the flow rate or pressure may be used in a feedback loop to control apump or other component controlling the actual coolant flow. However,depending on the thermal load experienced by a particular device as wellas the particular target temperature trying to be reached, there can besignificant thermal variations for a predetermined flow or pressurevalue at different portions of a medical device having fluid flowtherethrough. For example, for a fixed flow rate and a low thermal loadon a device, a flow rate may become excessive, resulting in a portion ofthe circulating fluid failing to change phase from a liquid/solid to agas, and thereby reducing the overall thermal efficiency and affect onthe surrounding tissue. Moreover, at a high heat load, a set flow ratemay not sufficiently provide a treatment area on the device having thedesired temperature, i.e., the temperature may vary drastically from onelocation to the next despite the proximity of the two locations becauseof the temperatures trying to be achieved and the thermal energy/loadsurrounding a particular device. As a result, the actual tip or devicetemperature may be different than a target temperature correlating to aset flow or pressure due to thermal variations at the treatment site.

Accordingly, it would be desirable to provide an improved apparatus andmethod of monitoring and controlling the circulation of a coolantthrough a medical device such as an intravascular catheter.

SUMMARY

The present invention advantageously provides a method and systemincluding a medical device having an injection lumen; a chamber in fluidcommunication with the injection lumen; an exhaust lumen in fluidcommunication with the chamber; a thermally-conductive elementcircumscribing (or otherwise disposed about) a substantial portion ofthe exhaust lumen proximal to the chamber; and a first temperaturesensor mounted on the thermally conductive element. Thethermally-conductive element can include at least one of a braid, coil,and band. The device may also include a second temperature sensorpositioned in the chamber, as well as a console in electricalcommunication with the first and second temperature sensors, thecontroller modifying coolant flow through the medical device based atleast in part upon a signal received from the first and/or secondtemperature sensor. The console may include a coolant supply in fluidcommunication with the injection lumen and a vacuum source in fluidcommunication with the exhaust lumen.

The present invention also includes an intravascular catheter system,including a catheter defining an injection lumen, a chamber in fluidcommunication with the injection lumen, and an exhaust lumen in fluidcommunication with the chamber; a first temperature sensor positioned inthe exhaust lumen proximal to the chamber; a second temperature sensorpositioned in the chamber; and a console in electrical communicationwith the first and second temperature sensors, the controller modifyingcoolant flow through the medical device based at least in part upon asignal received from the first and/or second temperature sensor. Athermally-conductive element may circumscribe a substantial portion ofthe exhaust lumen proximal to the chamber, and the first temperaturesensor may be mounted to the thermally-conductive element. The systemmay also include multiple temperature sensors coupled to multiplethermally-conductive elements positioned along a substantial length ofthe medical device.

The present invention also provides a method for controlling fluid flowthrough a medical device, including circulating a coolant through themedical device; measuring a first temperature at a distal portion of themedical device; measuring a second temperature at a position proximal ofthe distal portion of the medical device; and adjusting the circulationof coolant based at least in part upon the first and second temperaturemeasurements. The method may also include determining a pressuremeasurement or value based upon at least one of the first and secondtemperature measurements. Further, the first temperature measurement maybe performed by a first temperature sensor, and the second temperaturemeasurement may be performed by a second temperature, the method furtherincluding measuring an electrical resistance or impedance between thefirst and second temperature sensors; and determining the presence of aleak in the medical device based at least in part upon the measuredelectrical value or property. The method may also include thermallyaffecting a target tissue with the medical device, where thermallyaffecting the target tissue includes ablating cardiac tissue.

Presence of a leak may also be determined by measuring the temperatureusing one or more of the sensors described above. For example, liquidthat might enter the device evaporates under vacuum, thereby causing ameasurable temperature variation. As such, the method may also includecorrelating at least one of the first and second measured temperaturesto a leak condition of the medical device, and/or correlating adifference between the first and second measured temperatures to a leakcondition of the medical device. The correlation of a leak may be basedon one or more temperature measurements taken before, during, and/orafter any cooling fluid is introduced or otherwise circulated throughthe device.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is an embodiment of an exemplary medical treatment systemconstructed in accordance with the principles of the present invention;

FIG. 2 is a magnified image of a portion of the medical deviceillustrated in FIG. 1;

FIG. 3 is an illustration of a portion of an embodiment of a medicaldevice constructed in accordance with the principles of the presentinvention; and

FIG. 4 is an embodiment of an exemplary medical device constructed inaccordance with the principles of the present invention.

DETAILED DESCRIPTION

The present invention provides an improved apparatus and method ofmonitoring and controlling the circulation of a coolant through amedical device such as an intravascular catheter or surgical probe.Referring now to the drawing figures in which like reference designatorsrefer to like elements, there is shown in FIG. 1 an exemplary systemsuitable for thermally treating tissue, generally designated as 10. Thesystem 10 includes a console 12 coupled to a medical treatment device14, where the treatment device 14 may be a medical probe, a catheter, aballoon-catheter, as well as other devices commonly known in the art,such as devices able to pass easily through blood vessels and heartvalves and able to thermally affect tissue, for example. Of course, thepresent invention is compatible with catheters or probes that areequally adaptable for both endovascular and surgical proceduresinvolving thermal treatment applications.

In particular, the system 10 may include an elongate, highly flexiblecatheter that is suitable for passage through the vasculature. Themedical treatment device 14 may thus include a catheter body 16 having adistal end with a treatment element 18 or region at or proximal to thedistal end. The catheter body 16 may also define one or more lumensdisposed within the catheter body 16 thereby providing mechanical,electrical, and/or fluid communication between the proximal portion ofthe elongate body and the distal portion of the elongate body. Inaddition, the catheter body 16 may include a guidewire lumen (not shown)extending along at least a portion of the length of the catheter body 16for over-the-wire applications. The guidewire lumen may be movablydisposed within at least a portion of the catheter body 16 such that adistal end of the guidewire lumen extends beyond the and out of thedistal portion of the catheter body 16.

The catheter body 16 has a proximal end that is mated to a handle 20,and the handle 20 can include an element such as a lever or knob 22 formanipulating or deflecting at least a portion of the catheter body 16.In the exemplary embodiment, a pull wire having a proximal end and adistal end has its distal end anchored to the catheter body 16 and/ortreatment element 18 at or near the distal end. The proximal end of thepull wire is anchored to an element such as a cam 24 in communicationwith and responsive to the lever 22. The handle 20 can further includecircuitry 26 for identification and/or use in controlling of theablation catheter or another component of the system 10.

Continuing to refer to FIG. 1, the handle 20 can also include connectorsthat are matable directly or indirectly by way of one or more umbilicalsto the console 12 and a coolant or fluid supply 28, vacuum source unit30, and/or electronics therein. For example, in the illustrated system10, the handle 20 is provided with a first connector 32 that is matablewith a co-axial fluid umbilical (not shown) and a second connector 34that is matable with an electrical umbilical (not shown) that canfurther include an accessory box (not shown). The handle 20 may also beprovided with a fitting 36 for receiving a guide wire (not shown) thatis passed into the guide wire lumen. The handle 20 may also include oneor more pressure, temperature, flow rate, and/or other sensors tomonitor the desired characteristics and performance parameters of thesystem 10.

The console 12 provides the user interface to the system 10 and housesthe electronics and software for controlling and recording a selectedprocedure, controlling the delivery of the liquid refrigerant underpressure through the umbilical to the treatment device 14, controllingthe recovery of the expanded refrigerant vapor from the treatment device14 under vacuum, and for optionally controlling a compressor if presentto pressurize the coolant vapor into a liquid stored in a recovery tank(not shown).

As stated above, the console 12 may include the vacuum pump 30 or sourceunit in fluid communication with the medical treatment device 14. Thevacuum pump 30 is controllable to reduce the pressure within a portion,such as an exhaust fluid flow path, of the medical treatment device 14to provide a pressure ranging from a pure vacuum to a pressure justbelow a patient's blood pressure. For example, the vacuum pump 30 canmaintain a selected pressure between 80 mm Hg and 0 mm Hg prior toinjection. The provision of reduced pressure within a return flow pathof the catheter body 16 of the treatment device 14 significantlyenhances patient safety because, should a leak occur, refrigerant orcoolant will not flow from the device into the patient. Rather, bodilyfluids in the treatment site will be aspirated into the catheter body 16whereupon they may be sensed by leak detection elements and initiate acascade of pre-programmed events. In particular, either or both of thetreatment device 14 and the console 12 can be provided with detectiondevices that are in electrical communication with the console 12 andwhich may provide a signal output that can be representative of an eventthat indicates flow path integrity loss or a leak within a sealedportion of the surgical device and/or console 12. The console 12 can beconfigured to respond to signal output from the one or more sensors ordetectors and initiate a predetermined sequence of events, such asdiscontinuing refrigerant injection, changing the pressure or flow ratewithin the system 10, and/or controlling removal of refrigerant from thecatheter body 16.

In addition to providing an exhaust function for the fluid in themedical treatment device 14, the console 12 can also recover and/orrecirculate the cooling fluid. In addition, console 12 can include anLCD touch screen that displays console status and data, and accepts userdata and control inputs. Various discrete indicators, controls, anddisplays may also be included which indicate the status of one or moreconsole parameters and allow inputs for manual system 10 operation.

Now referring to FIGS. 1 and 2, the distal end of the medical treatmentdevice 14 is shown. The distal end or region of the medical device mayinclude the thermally-transmissive or conductive treatment element 18coupled to an insulated or otherwise less thermally-conductive shaftportion 38 of the medical device body 16. For example, the distal regionmay include a metallic tip, a conductive balloon (not shown) or the likedefining a chamber receiving coolant flow for enhanced thermalinteraction with a target tissue. A coolant supply tube 40 defining afluid supply lumen may be included in fluid communication with thecoolant supply 28 in the console 12. The coolant supply tube 40 mayinclude one or more openings able to disperse a provided fluid orcoolant within and/or proximate to the distal region and the treatmentelement 18 of the device 14 in response to console 12 commands and othercontrol input. The medical treatment device 14 may further include acoolant exhaust or return lumen 42, which may be defined at least inpart by the medical device body 16. The vacuum pump 30 in the console 12may be in fluid communication with the exhaust/return lumen 42 to createa pressure gradient within the medical device 14 so that coolant isdrawn away from the distal tip and toward the proximal end of themedical device body 16.

The distal region of the medical device 14 may further include one ormore reinforcement or structural elements 44 such as coils, braids,rings, or the like embedded, positioned, and/or otherwise coupled to aportion of the medical device body. The reinforcement elements 44 may bedirectly exposed to or otherwise be in thermal communication with theexhaust lumen 42 of the medical device 14, and may be constructed from ametallic material for example. The reinforcement elements 44 maycircumscribe all of and/or a substantial portion of the circumference ofthe medical device body 14, and may further extend along a substantiallength of the medical device body 16 from a position just proximal to oradjacent to the treatment element 18 to a proximal end of the devicenear the handle 20.

The medical treatment device 14 may further include one or moretemperature sensors positioned in thermal communication with the exhaustlumen, and in particular, may be exposed to fluid flow therethrough orotherwise in thermal communication with the thermally-transmissiveelement(s). The temperature sensors may include thermocouples,thermistors, or other temperature-sensitive device or element as knownin the art. For example, the medical treatment device 14 may include afirst temperature sensor 46 a mounted on or otherwise connected to areinforcement element 44 such as conductive band or ring circumscribingat least a portion of the circumference of the exhaust lumen 42 in aregion of the medical device body 16 proximal to the treatment element18, where the conductive band/ring may be constructed from a metallic orotherwise thermally-conductive material. Alternatively, the firsttemperature sensor 46 a may be mounted on or otherwise connected to oneor more of the reinforcement elements 44 such as a coil or braidpositioned along a length of the medical device body proximal to thetreatment element 18.

The medical device 14 may further include one or more temperaturesensors such as a thermocouple, thermistor, or othertemperature-sensitive device or element disposed within or otherwisemounted on the treatment element 18. For example, a second temperaturesensor 46 b may be positioned adjacent to one or more openings in thecoolant supply tube 40 for measurement and/or monitoring of thermalcharacteristics in the region. The one or more temperature sensors ofthe medical device 14 may be electrically coupled to or otherwise incommunication with the console 12 for sending signals corresponding tomeasured or sensed temperatures thereto, which may impact or otherwiseaffect control of fluid flow therethrough.

Now referring to FIGS. 3 and 4, the medical treatment device 14 mayinclude multiple temperature sensors 46 c, 46 d, 46 e . . . etc.(collectively referred to as “46”) disposed along the length of themedical device 14 and/or catheter body 16, where the multipletemperature sensors 46 are intermittently positioned to provideinformation about the thermal characteristics and environment atdiscrete locations of the medical device 14 in order to more effectivelycontrol cooling and treatment. For example, as shown in FIG. 3, themultiple temperature sensors 46 may be coupled to or otherwise mountedon intersecting segments of a support element 44 such as a braid, whichas discussed above, may be thermally conductive and constructed from ametallic material. As shown in FIG. 4, the temperature sensors 46 may becoupled to reinforcement elements 44 such as rings or bandsintermittently located along the length of the medical device body 16 tosimilarly provide information at discrete locations of the device. Asdescribed above, each of the temperature sensors may be electricallycoupled to and/or otherwise in communication with the console 12 to aidin regulating and controlling characteristics of coolant flow throughthe medical treatment device 14.

In an exemplary method of use for the system 10 described above andshown in FIGS. 1-4, a user may employ the system 10 for a thermaltreatment of desired tissue. For example, an ablation and/or mappingprocedure on cardiac tissue may be desirable, and as such, may beexecuted through manipulation of the medical treatment device 14 and thecontrols of the console 12. In particular, the treatment may include theinitiation of coolant flow through the medical device and the treatmenttip, which may be achieved through pressurization of the coolant formthe coolant supply and/or manipulation of the operation of the vacuumpump to provide the desired flow rate, pressure level, and/or resultingtemperature characteristics at the tip. The coolant may include acombination of various gases and liquids including but not limited toargon, carbon dioxide, nitrous oxide, liquid nitrogen or the like.Should pressurized gas or liquid be delivered, the coolant may becomedepressurized and/or otherwise expand in the treatment tip, and aresulting phase-change provides a reduced temperature for thermallyaffecting the tissue.

During the circulation of coolant through the medical treatment device14, measured temperature values from at least the first and secondtemperature sensors located in the exhaust path 42 and the treatmentelement 18, respectively, may be relayed or otherwise monitored at theconsole 12. As discussed above, there may be significant discrepanciesbetween a temperature in the treatment element 18 where the coolantexpansion may occur, and a portion of the exhaust path 42 just proximalof the distal treatment tip. For example, it has been found thattemperature differences between the tip where the coolant expansion mayoccur and a portion of the exhaust path 42 just fractions of an inchproximal of the treatment element 18 can be as much as twenty degreesCelsius or more. The significant discrepancies may result in a failureof the coolant to provide optimal cooling in the thermal interactionbetween the treatment device 14 and the target tissue.

The resulting signals conveyed to the console 12 by the two temperaturesensors may be used to adjust, regulate, or otherwise control fluid flowthrough the treatment device 14. For example, if it is found that thetemperature in the tip is sufficient for treatment, but the temperaturein the return path 42 is too low and/or the coolant is not completelychanging phase, the flow rate and/or pressure of the coolant in themedical treatment device 14 may be adjusted accordingly at or by theconsole 12. By adjusting a flow rate and/or pressure of the coolantcirculation based upon multiple temperature sensor signals, a moreaccurate and efficient cooling treatment process may be achieved withoutwasting coolant or otherwise failing to optimally deliver the desiredthermal treatment.

In addition to monitoring signals from the first and second temperaturesensors to adjust flow, in an embodiment where there are a plurality oftemperature sensors 46 intermittently disposed along the length of theshaft, as discussed above with respect to FIGS. 3 and 4, the signalsfrom the plurality of sensors may be used to map or otherwise monitorthermal characteristics occurring at discrete positions along the lengthof the device 14, and flow rates/pressures may be adjusted accordingly.The temperature sensors provide a more complete picture of the thermalbehavior of the device and the coolant circulation path under varyingheat loads. For example, depending on the length of a particularcatheter and/or the depth of insertion into a patient, the thermal loadswill vary significantly along the length of the catheter body, i.e., theportion of the device outside of the body versus the portion inside, andthese variations may be monitored and provide a basis for adjustmentduring a treatment procedure.

Of note, the output or measurement signals of the one or moretemperature sensors 46 of the medical treatment device may also be usedto determine a pressure of coolant at that particular point in thedevice. In particular, a temperature value of a fluid at saturation canbe directly correlated to a pressure. For example, if particularphysical properties of the coolant being used are known, then atemperature measurement of that substance under saturation can be usedto determine a resulting pressure level. As such, one or moretemperature sensors may be able to provide the function of deliveringboth a direct temperature reading as well as forming the basis for acalculated pressure value given known physical properties of thecoolant. In a particular example, a saturation pressure is proportionalto the saturation temperature.

In an additional method of use for the system and components describedabove, two or more of the temperature sensors 46 may be employed toprovide for the detection and resulting indication of a leak in aportion of the medical device. In particular, two of the temperaturesensors, which may include thermocouples, may be electrically isolatedand have a common ground. In addition to monitoring the signalsindicative of temperature measurements by the temperature sensors, aresistance or impedance between the two temperature sensors can bemonitored and changes thereof can indicate a leak. In particular, when aleak occurs and fluid form the surrounding tissue is entrained into theexhaust path (which is under vacuum). The entrained liquid mayevaporate, and thus cause localized cooling, which can be indicated by atemperature sensor in proximity to the leak by having a different valuethan the surrounding sensors. In addition, the entrained fluid changesthe electrical value between two or more of the temperature sensors, andthus the change in resistance, impedance or the like can also indicatethe presence of a leak or breach of the structural integrity of thefluid flow path.

By employing the existing or present temperature sensors to detect aleak, there is no need for additional leak detection apparatus, whichmay take up space in a small dimensioned medical device, add tomanufacturing cost, etc. Moreover, monitoring temperature provides aleak detection apparatus that does not require the use of high frequencycircuits as with impedance leak detection circuit schemes presentlyknown in the art. In addition, the cables or wires of the temperaturesensors may be braided or shielded to eliminate electrical cross-talk orinterference with the surrounding environment, such as a radiofrequencygenerator used in the vicinity.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed herein above. In addition, unless mention was made above tothe contrary, it should be noted that all of the accompanying drawingsare not to scale. A variety of modifications and variations are possiblein light of the above teachings without departing from the scope andspirit of the invention, which is limited only by the following claims.

What is claimed is:
 1. A method for controlling fluid flow through amedical device, comprising: circulating a coolant through the medicaldevice; measuring a first temperature at a distal portion of the medicaldevice; measuring a second temperature at a position proximal of thedistal portion of the medical device; and adjusting the circulation ofcoolant based at least in part upon the first and second temperaturemeasurements.
 2. The method of claim 1, further comprising determining apressure measurement based upon at least one of the first and secondtemperature measurements.
 3. The method of claim 1, wherein the firsttemperature measurement is performed by a first temperature sensor, andthe second temperature measurement is performed by a second temperature,the method further comprising: measuring an electrical resistancebetween the first and second temperature sensors; and determining thepresence of a leak in the medical device based at least in part upon themeasured electrical resistance.
 4. The method of claim 3, wherein themedical device includes a treatment element defining a chamber.
 5. Themethod of claim 4, wherein the first temperature sensor is located inthe chamber.
 6. The method of claim 4, wherein the medical deviceincludes an exhaust lumen located proximal to the treatment element. 7.The method of claim 6, wherein the second temperature sensor is locatedin the exhaust lumen.
 8. The method of claim 7, wherein the medicaldevice includes a thermally conductive support element located entirelywithin the exhaust lumen, the second temperature sensor being mounted onthe support element.
 9. The method of claim 7, wherein the firsttemperature sensor and the second temperature sensor are electricallyisolated from each other.
 10. The method of claim 1, wherein themeasurement of the first temperature occurs at a treatment element onthe distal portion of the medical device.
 11. The method of claim 1,wherein the measurement of the second temperature occurs in an exhaustlumen of the medical device proximal to the treatment element.
 12. Themethod of claim 1, further comprising thermally affecting a targettissue with the medical device.
 13. The method of claim 1, whereinthermally affecting the target tissue includes ablating cardiac tissue.14. The method according to claim 1, further comprising correlating atleast one of the first and second measured temperatures to a leakcondition of the medical device.
 15. The method of claim 1, furthercomprising correlating a difference between the first and secondmeasured temperatures to a leak condition of the medical device.
 16. Themethod of claim 1, wherein the first temperature measurement isperformed by a first temperature sensor, and the second temperaturemeasurement is performed by a second temperature, the method furthercomprising: measuring an electrical impedance between the first andsecond temperature sensors; and determining the presence of a leak inthe medical device based at least in part upon the measured electricalimpedance.
 17. The method of claim 16, wherein the medical deviceincludes a treatment element defining a chamber, an exhaust lumenlocated proximal to the treatment element, and a thermally conductivesupport element located entirely within the exhaust lumen, the firsttemperature sensor being located within the chamber and the secondtemperature sensor being mounted on the support element.
 18. A methodfor controlling fluid flow through a medical device, comprising:circulating a coolant through the medical device; measuring a firsttemperature with a first temperature sensor, the first temperaturesensor being located within a treatment element coupled to the distalportion of the medical device; measuring a second temperature with asecond temperature sensor, the second temperature sensor being mountedon a support element that is located entirely within an exhaust lumenlocated proximal to the treatment element; and adjusting the circulationof coolant based at least in part upon the first and second temperaturemeasurements.
 19. The method of claim 18, wherein the support element isthermally conductive.
 20. The method of claim 19, wherein the supportelement comprises a plurality of support elements.